Rusticl: make it even simpler for drivers to support OpenCL
The promise of rusticl was always that it should be as simple as possible to bring OpenCL to gallium drivers, this improves the situation a bit further:
- no need for handling 64 bit global invocation ids (!24905 (merged) started it, but here I extend it a little)
- no need to support vec8/16 in backend compilers anymore! (doesn't work for vec4 backends yet)